The use of protection devices capable of protecting electrical or electronic apparatuses from overvoltages, which may result, for example, from lightning, is known.
These protection devices generally comprise one or more overvoltage protection components, such as, for example, a varistor or a spark gap. When the protection components are exposed to voltages above a predetermined threshold, the protection components are capable of conducting the default current to the ground while limiting the overvoltage to a value compatible with the capacity of the installation and equipment connected to the protection device.
In the event of a failure, protection components may be subject to significant heating that can cause serious damage to the installation and present risks to the user, for example, by starting a fire. This is why overvoltage protection devices are generally provided with thermal disconnection means. These thermal disconnection means are intended to isolate the protection component from the electrical installation to be protected in the case of excessive heating of the component to suppress the electrical power supply causing the heating and prevent the appearance or limit the harmful consequences of an excessive increase in temperature.
To make the thermal disconnection means sensitive to the increase in temperature of the protection component, it is known to use a fuse element of which the state is capable of being modified by the heat released by the protection component. In particular, it is known to use a solder fuse to produce the connection between one of the power supply terminals of the protection component and a mobile conductor element, such as a slider or a disconnection leaf, which is generally subjected to mechanical stress tending to separate it from the power supply terminal. Thus, when excessive heating of the protection component causes the solder to melt, the mobile conductive element separates from the power supply terminal of the protection component, thus isolating the power supply terminal from the installation to be protected.
To prevent dangerous heating of the component, the disconnection should occur before the temperature reaches a critical threshold. To this end, the fuse elements are generally produced using specific metal alloys with a relatively low melting point. Normally, the alloys used for this purpose contain lead and often other toxic materials such as cadmium.
However, health regulations will soon prohibit the use of certain toxic materials, in particular lead, as a material constituting electrical and electronic products.
It may be necessary to stop using lead alloys previously used for producing the fuse elements of thermal disconnection means, and instead use lead-free alloys with a melting temperature that is at least as low.
While such alloys are known, such as, for example, certain tin-bismuth alloys, it is impossible to replace fuse elements in the same state in the existing overvoltage protection devices because the known lead-free alloys do not have the same mechanical properties, and particularly do not have the same stress resistance as the lead alloys.
In particular, this lower mechanical strength exposes the weld joint to premature rupture capable of occurring under the effect of the electrodynamic stresses exerted on the conductive elements of the protection device when traversed by a discharge current corresponding to the limitation of an overvoltage in the context of its normal operation. In other words, the use of a lead-free solder fuse is capable of reducing the discharge power of the protection device, i.e., damaging its performance by decreasing the maximum intensity of the default current that the device can conduct repeatedly without undergoing damage.
Two major difficulties are encountered in the replacement of lead fuse elements with lead-free fuse elements. The first major difficulty is maintaining a low enough melting point to ensure the safety of operation, i.e., a fast enough activation of the thermal disconnection means; and the second major difficulty is to have adequate mechanical strength in the connection produced by the fuse element, in particular so as not to disturb the normal operation of the device by an inappropriate disconnection during the conduction of a discharge current.